1. Field of the Invention
The present invention relates to magnet structures and more particularly to magnet structures for use with magnetically coupled hearing aids.
2. Description of the Prior Art
Conventional hearing aids utilize detection, amplification and retransmission of the acoustic waves forming sound. Because of a number of well known problems with conventional hearing aids, magnetically coupled hearing aids were investigated. In a magnetically coupled hearing aid a magnet or magnetic material is placed in the middle ear so that any movement of the magnet structure is perceived as sound by the wearer. The hearing aid includes a coil used to develop a magnetic field which is coupled to the magnetic field produced by the magnetic material. The coil magnetic field is varied based on the received sound waves, with the coupling between the two fields causing the magnetic material to vibrate in sympathy. This motion of the magnetic material then vibrates the connected portion of the middle ear and sound is perceived by the wearer.
Because these magnetic hearing aids are electrically powered, generally by very small batteries, overall efficiency is critical with the highest possible efficiency being desirable. The increased power consumption that is necessitated if the electrical circuitry utilizes too much power itself or if the coupling between the magnetic fields is poor may reduce efficiency of the hearing aid, and therefore the battery life, to unacceptable limits. Because of the state of current electronics, the most promising area for improvement is the coupling of the magnetic fields.
While increasing the size and therefore field strength of the implanted magnetic material is a possibility to improve magnetic field coupling, the larger amount of magnetic material is also increasingly vulnerable to external magnetic fields. For example, if the user is too close to the external magnetic field from an electrical transformer, a 60 Hz hum may be developed by the coupling of the magnetic material magnetic field and the transformer magnetic field. This is a drawback to simply increasing the size of the magnetic material and is an effect which is desirable to limit.
The coupling could be increased by increasing the strength of the magnetic field output by the hearing aid coil. One way to increase this field is to increase the current in the coil, thereby increasing the ampere-turns value. This increase is practical only within given limits because the increase in current directly affects battery life. Increasing the number of turns is also possible, but again has practical limitations. Because of the limited volume that can be occupied by the coil, especially if the coil is located in the ear canal, the number of turns can only be increased by reducing the size of the wire forming the coil. However, as this wire size is reduced, its unit resistance, and therefore overall coil resistance, increases. Because the amplifier driving the coil is customarily a voltage source, it is sensitive to this output load and the current provided to the coil can reduce as the resistance increases. Therefore, there are only limited gains to be obtained by changing the coil current or number of turns. Gains must be developed in a manner other than simply increasing ampere-turns value.
The coil could be placed closer to the magnetic material, but given the size of the hearing aid components and the vulnerability of the middle ear, certain effective minimum spacings are necessary, particularly if the extended surgery that may be necessary for very close implantation is not desirable or possible. Additionally, it is desirable that as much of the hearing aid as possible is easily removable, to limit surgical problems and to ease repair and replacement of the hearing aid and its battery. This removability, when coupled with the physical sizes of the hearing aid components, limits the attainable distance between the coil and magnetic material.